Astm c 1402 17

Designation C1402 − 17 Standard Guide for High Resolution Gamma Ray Spectrometry of Soil Samples1 This standard is issued under the fixed designation C1402; the number immediately following the design[.]

Designation: C1402−17

Standard Guide for

This standard is issued under the fixed designation C1402; the number immediately following the designation indicates the year of

original adoption or, in the case of revision, the year of last revision A number in parentheses indicates the year of last reapproval A

superscript epsilon (´) indicates an editorial change since the last revision or reapproval.

1 Scope

1.1 This guide covers the identification and quantitative

determination of gamma-ray emitting radionuclides in soil

samples by means of gamma-ray spectrometry It is applicable

to nuclides emitting gamma rays with an approximate energy

range of 20 to 2000 keV For typical gamma-ray spectrometry

systems and sample types, activity levels of about 5 Bq (135

pCi) are measured easily for most nuclides, and activity levels

as low as 0.1 Bq (2.7 pCi) can be measured for many nuclides

It is not applicable to radionuclides that emit no gamma rays

such as the pure beta-emitting radionuclides hydrogen-3,

carbon-14, strontium-90, and becquerel quantities of most

transuranics This guide does not address the in situ

measure-ment techniques, where soil is analyzed in place without

sampling Guidance for in situ techniques can be found in Ref

( 1 ) and ( 2 ).2 This guide also does not discuss methods for

determining lower limits of detection Such discussions can be

found in Refs (3 ), ( 4 ), ( 5 ), and ( 6 ).

1.2 This guide can be used for either quantitative or relative

determinations For quantitative assay, the results are expressed

in terms of absolute activities or activity concentrations of the

radionuclides found to be present This guide may also be used

for qualitative identification of the gamma-ray emitting

radio-nuclides in soil without attempting to quantify their activities

It can also be used to only determine their level of activities

relative to each other but not in an absolute sense General

information on radioactivity and its measurement may be

found in Refs (7 ), ( 8 ), ( 9 ), ( 10 ), and ( 11 ) and Standard Test

MethodsE181 Information on specific applications of

gamma-ray spectrometry is also available in Refs (12 ) or ( 13 ) Practice

D3649may be a valuable source of information

1.3 The values stated in SI units are to be regarded as

standard No other units of measurement are included in this

standard

1.4 This standard may involve hazardous material,

operations, and equipment This standard does not purport to

address all of the safety concerns, if any, associated with its use It is the responsibility of the user of this standard to establish appropriate safety and health practices and deter-mine the applicability of regulatory limitations prior to use.

1.5 This international standard was developed in

accor-dance with internationally recognized principles on standard-ization established in the Decision on Principles for the Development of International Standards, Guides and Recom-mendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

2 Referenced Documents

2.1 ASTM Standards:3

C859Terminology Relating to Nuclear Materials

C998Practice for Sampling Surface Soil for Radionuclides

C999Practice for Soil Sample Preparation for the Determi-nation of Radionuclides

C1009Guide for Establishing and Maintaining a Quality Assurance Program for Analytical Laboratories Within the Nuclear Industry

D3649Practice for High-Resolution Gamma-Ray Spectrom-etry of Water

D7282Practice for Set-up, Calibration, and Quality Control

of Instruments Used for Radioactivity Measurements

E181Test Methods for Detector Calibration and Analysis of Radionuclides

IEEE/ASTM-SI-10Standard for Use of the International System of Units (SI) the Modern Metric System

2.2 ANSI Standards:4

N13.30Performance Criteria for Radiobioassay

N42.14Calibration and Use of Germanium Spectrometers for the Measurement of Gamma-Ray Emission Rates of Radionuclides

N42.23American National Standard Measurement and As-sociated Instrumentation

IEEE-325Standard Test Procedures for Germanium Gamma-Ray Detectors

1 This guide is under the jurisdiction of ASTM Committee C26 on Nuclear Fuel

Cycle and is the direct responsibility of Subcommittee C26.05 on Methods of Test.

Current edition approved June 1, 2017 Published July 2017 Originally approved

in 1998 Last previous edition approved in 2009 as C1402 – 04 (2009) DOI:

10.1520/C1402-17.

2 The boldface numbers in parentheses refer to the list of references at the end of

this standard.

3 For referenced ASTM standards, visit the ASTM website, www.astm.org, or

contact ASTM Customer Service at service@astm.org For Annual Book of ASTM

Standards volume information, refer to the standard’s Document Summary page on

the ASTM website.

4 Available from American National Standards Institute (ANSI), 25 W 43rd St., 4th Floor, New York, NY 10036, http://www.ansi.org.

Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959 United States

3 Terminology

3.1 Except as otherwise defined herein, definitions of terms

are as given in Terminology C859

4 Summary of Guide

4.1 High-resolution germanium detectors and multichannel

analyzers are used to ensure the identification of the

gamma-ray emitting radionuclides that are present and to provide the

best possible accuracy for quantitative activity determinations

4.2 For qualitative radionuclide identifications, the system

must be energy calibrated For quantitative determinations, the

system must also be shape and efficiency calibrated The

standard sample/detector geometries must be established as

part of the efficiency calibration procedure

4.3 The soil samples typically need to be pretreated (for

example, dried), weighed, and placed in a standard container

For quantitative measurements, the dimensions of the container

holding the sample and its placement in front of the detector

must match one of the efficiency-calibrated geometries If

multiple geometries can be selected, the geometry chosen

should reflect the detection limit and count rate limitations of

the system Qualitative measurements may be performed in

non-calibrated geometries

4.4 The identification of the radionuclides present is based

on matching the energies of the observed gamma rays in the

spectrum to computer-based libraries of literature references

[see Refs (14 ), ( 15 ), ( 16 ), ( 17 ), or ( 18 )] The quantitative

determinations are based on comparisons of observed count

rates to previously obtained counting efficiency versus energy

calibration data, and published branching ratios for the

radio-nuclides identified

5 Significance and Use

5.1 Gamma-ray spectrometry of soil samples is used to

identify and quantify certain gamma-ray emitting

radionu-clides Use of a germanium semiconductor detector is

neces-sary for high-resolution gamma-ray measurements

5.2 Much of the data acquisition and analysis can be

automated with the use of commercially available systems that

include both hardware and software For a general description

of the typical hardware in more detail than discussed in Section

7, see Ref (19 ) For best practices on set-up, calibration, and

quality control of utilized spectrometry systems, see Practice

D7282

5.3 Both qualitative and quantitative analyses may be

per-formed using the same measurement data

5.4 The procedures described in this guide may be used for

a wide variety of activity levels, from natural background

levels and fallout-type problems, to determining the

effective-ness of cleanup efforts after a spill or an industrial accident, to

tracing contamination at older production sites, where wastes

were purposely disposed of in soil In some cases, the

combination of radionuclide identities and concentration ratios

can be used to determine the source of the radioactive

materials

5.5 Collecting samples and bringing them to a data acqui-sition system for analysis may be used as the primary method

to detect deposition of radionuclides in soil For obtaining a representative set of samples that cover a particular area, see Practice C998 Soil can also be measured by taking the data acquisition system to the field and measuring the soil in place (in situ) In situ measurement techniques are not discussed in this guide

6 Interferences

6.1 In complex mixtures of gamma-ray emitters, the degree

of interference of one nuclide in the determination of another

is governed by several factors Interference will occur when the photopeaks from two separate nuclides overlap within the resolution of the gamma-ray spectrometer Most modern analy-sis software can deconvolute multiplets where the separation of

any two adjacent peaks is more than 0.5 FWHM (see Refs (20 ) and (21 )) For peak separations that are smaller than 0.5

FWHM, most interference situations can be resolved with the

use of automatic interference correction algorithms (22 ).

6.2 If the nuclides are present in the mixture in very unequal radioactive portions and if nuclides of higher gamma-ray energy are predominant, the interpretation of minor, less energetic gamma-ray photopeaks becomes difficult due to the high Compton continuum and backscatter

6.3 True coincidence summing (also called cascade sum-ming) occurs regardless of the overall count rate for any radionuclide that emits two or more gamma rays in coinci-dence Cobalt-60 is an example where both a 1173-keV and a 1332-keV gamma ray are emitted from a single decay If the sample is placed close to the detector, there is a finite probability that both gamma rays from each decay interact within the resolving time of the detector resulting in a loss of counts from both full energy peaks Coincidence summing and the resulting losses to the photopeak areas can be considerable (>10 %) before a sum peak at an energy equal to the sum of the coincident gamma-ray energies becomes visible Coincidence summing and the resulting losses to the two individual photo-peak areas can be reduced to the point of being negligible by increasing the source to detector distance or by using a small detector Coincidence summing can be a severe problem if a well-type detector is used See Test MethodsE181and (7 ) for

more information

6.4 Random summing is a function of count rate (not dead time) and occurs in all measurements The random summing rate is proportional to the total count squared and to the resolving time of the detector and electronics For most systems, uncorrected random summing losses can be held to less than 1 % by limiting the total counting rate to less than

1000 counts/s However, high-precision analyses can be per-formed at high count rates by the use of pileup rejection circuitry and dead-time correction techniques Refer to Test Methods E181for more information

7 Apparatus

7.1 Germanium Detector Assembly—The detector should

have an active volume of greater than 50 cm3, with a full width

at one half the peak maximum (FWHM) less than 2.0 keV for

the cobalt-60 gamma ray at 1332 keV, certified by the

manufacturer A charge-sensitive preamplifier should be an

integral part of the detector assembly

7.2 Sample Holder Assembly—As reproducibility of results

depends directly on reproducibility of geometry, the system

should be equipped with a sample holder that will permit using

reproducible sample/detector geometries for all sample

con-tainer types that are expected to be used at several different

sample-to-detector distances

7.3 Shield—The detector assembly should be surrounded by

a radiation shield made of material of high atomic number

providing the equivalent attenuation of 100 mm (or more in the

case of high background radiation) of low-activity lead It is

desirable that the inner walls of the shield be at least 125 mm

distant from the detector surfaces to reduce backscatter and

annihilation radiation If the shield is made of lead or has a lead

liner, the shield should have a graded inner shield of

appropri-ate mappropri-aterials, for example, 1.6 mm of cadmium or tin-lined

with 0.4 mm of copper, to attenuate the induced 88-keV lead

fluorescent X-rays The shield should have a door or port for

inserting and removing samples The materials used to

con-struct the shield should be prescreened to ensure that they are

not contaminated with unacceptable levels of natural or

man-made radionuclides The lower the desired detection capability,

the more important it is to reduce the background For very low

activity samples, the detector assembly itself, including the

preamplifer, should be made of carefully selected low

back-ground materials

7.4 High-Voltage Power/Bias Supply—The bias supply

re-quired for germanium detectors usually provides a voltage up

to 65000 V and 1 to 100 µA The power supply should be

regulated to 0.1 % with a ripple of not more than 0.01 % Noise

caused by other equipment should be removed with r-f filters

and power line regulators

7.5 Amplifier—A spectroscopy amplifier which is

compat-ible with the preamplifier If used at high count rates, a model

with pile-up rejection should be used The amplifier should be

pole-zeroed properly prior to use

7.6 Data Acquisition Equipment—A multichannel

pulse-height analyzer (MCA) with a built-in or stand-alone

analog-to-digital converter (ADC) compatible with the amplifier

output and pileup rejection scheme The MCA (hardwired or a

computer-software-based) collects the data, provides a visual

display, and stores and processes the gamma-ray spectral data

The four major components of an MCA are: ADC, memory,

control, and input/output The ADC digitizes the analog pulses

from the amplifier The height of these pulses represents energy

deposited in the detector The digital result is used by the MCA

to select a memory location (channel number) which is used to

store the number of events which have occurred at the energy

The MCA must also be able to extend the data collection time

for the amount of time that the system is dead while processing

pulses (live time correction)

7.7 Count Rate Meter—It is useful but not mandatory to

have a means to measure the total count rate for pulses above

the amplifier noise during the measurement If not provided by

the MCA, a separate count rate meter may be used for this purpose In the absence of a rate meter, count rates that are too high to provide reliable results may also be detected by monitoring the system dead time or peak resolution, or both

7.8 Pulser—Required only if random summing effects are

corrected with the use of a stable pulser (23 ) and ( 24 ).

7.9 Computer—Most modern gamma-ray spectrometers are

equipped with a computer for control of the data acquisition as well as automated analysis of the resulting spectra Such computer-based systems are readily available from several commercial vendors Their analysis philosophies and capabili-ties do differ from each other somewhat See ANSI N42.14 for

a series of tests on how to tell if a particular gamma-spectrometry software package has adequate analysis capabili-ties In addition to the analysis capabilities, it is important to consider the overall user interface and architecture of the software For small-scale operations, a few samples per week,

a user interface that requires a lot of user intervention is sufficient For larger-scale operations, with hundreds of samples per week on multiple detectors, a software package that permits some kind of batch processing and automated operation is recommended

8 Container for a Test Sample

8.1 Sample holders and containers must have a reproducible geometry Considerations include commercial availability, ease

of use and disposal, and the containment of radioactivity for protection of the working environment, personnel, and the gamma-ray spectrometer from contamination For small soil samples (up to a few grams), plastic bottles are convenient containers, while large samples (up to several kilograms), which require greater sensitivity, are frequently packaged in Marinelli beakers For analyzing low-energy gamma rays at close geometries, the consistency of the wall thickness of the sample container facing the detector becomes an important factor in the variability of the analysis results

8.2 Measurements may require precautions to prevent the loss of volatile radionuclides For example, the direct determi-nation of radium-226 in soil by the measurement of the 609-keV gamma ray of bismuth-214 assumes secular equilib-rium between radium-226 and its bismuth-214 progency and that the radon-222 daughter was not lost from the sample 8.3 A beta absorber consisting of about 6 mm of aluminum, beryllium, or plastic should be placed between the detector and sample for samples that have significant quantities of high-energy beta emitters

9 Calibration and Standardization

9.1 Overview:

9.1.1 Commission and operate the instrumentation and de-tector in accordance with the manufacturer’s instructions and best practices such as may be contained in Practice D7282 Initial set-up includes all electronic adjustments to provide constant operating conditions consistent with the application and life expectancy of the calibrations The analog-to-digital converter gain and range, amplifier gain, and zero-level must

be adjusted to yield an optimum energy calibration Both the

energy and efficiency calibration must be accomplished with

radioactive sources covering the entire energy range of interest

(6, 7 and Test MethodsE181) Subsequent efficiency

calibra-tions and source analyses are performed with the same gain

settings and the same high-voltage setting Prepare efficiency

calibration standards by weighing an appropriate amount of a

radionuclide standard solutions containing 100 to 10 000 Bq

each onto a soil matrix in an appropriate container, drying it,

and mixing thoroughly Standardized dried soil and bottom

sediment are also available from the U.S National Institute of

Standards and Technology (NIST) or other appropriate sources

which can be used directly or diluted with ambient soil to a

measured weight or volume Prepare blank sources containing

the same quantity of unspiked soil to account for any naturally

occurring radionuclides that may be present Commercially

available epoxy soil-equivalent standards with an appropriate

mixture of radionuclides can also be used It should be noted

that soils that contain high atomic number materials will

significantly alter the expected self-attenuation

9.1.2 Follow the manufacturer’s instructions, limitations,

and cautions for the setup and the preliminary testing for all of

the spectrometry equipment to be used in the analysis This

equipment would include, as applicable, detector, power

supplies, preamplifiers, amplifiers, multichannel analyzers, and

computing systems For example, ensure that the detector has

had ample time (typically 6–8 h) to cool down after the first

filling with liquid nitrogen before turning on the high voltage

Also, ensure that the high-voltage bias supply is set for the

recommended operating voltage and the correct polarity

9.1.3 Place an appropriate weight of standardized dried soil

in an appropriate soil matrix in a sealed container and place the

container at a desirable and reproducible source-to-detector

distance The standard (traceable to a designated standards

organization) should provide enough counts in each calibration

peak (typically 20 000 or more, see Test Methods E181 or

ANSI N42.14) in a reasonable amount of time (4–12 h) In all

radionuclide measurements, the volumes, shape, and physical

and chemical characteristics of all the samples and standards

and their containers must be as identical as practical for the

most accurate results For situations where it is not possible or

practical to produce standards that are identical to the samples,

standard matrices that are different from the sample matrices

have been found to provide acceptable results when coupled

with attenuation correction methods

9.2 Energy and Shape Calibration:

9.2.1 The energy and shape calibration (the peak

gamma-ray energy versus channel number of the multichannel analyzer

and peak shape versus the peak gamma-ray energy) of the

detector system is determined at a specific gain setting

(typi-cally 0.5 keV/channel) using standards containing known

radionuclides The peak shape calibration may involve only

calculating the peak resolution (full-width-at-half-maximum,

or FWHM), or include other, nonsymmetrical components as

well The standards should be in sealed containers and should

emit at least eight different gamma-ray energies covering the

range of interest, usually from 20 to 2000 keV, in order to test

for system linearity If the calibration is performed with only

the radionuclides of interest, fewer gamma-ray energies can

also be used Energy and shape calibration can be performed without NIST traceable sources

9.2.2 Verify the radionuclide purity of the standard periodi-cally to ensure against accidental contamination or the pres-ence of long-lived impurities by comparing the observed gamma rays with the data published in the literature Careful adherence to precautions and certificate calibration instructions are necessary when using the calibration standards

9.2.3 Calibrate a multichannel analyzer for energy, shape, and efficiency to cover the energy range or interest If the range

of interest is from 20 to 2000 keV, adjust the gain of the system until the centroid of the cesium-137 photopeak, 661.6 keV, is about one-third full-scale Leaving the gain constant, locate at least three other photopeaks of different energies within the energy range of interest Determine and record the peak centroid for each of the four gamma energies A linear relationship between the gamma-ray energies and their channel numbers should be observed if the equipment is operating properly Calculate the slope and intercept of the line using a least-squares calculation If the spectrometry system is computerized, follow the appropriate manufacturer input in-structions for the determination of the slope and intercept 9.2.4 If the system is being calibrated with the radionuclides

of interest, fewer lines may be used for calibration and the linearity of the MCA is not an issue as long as the peaks of interest are identified and quantified consistently

9.3 Effıciency Calibration:

9.3.1 Efficiency calibration must be performed with sources that are traceable to a national standards laboratory, such as NIST A mixed gamma-ray standard for both energy and efficiency calibration containing Am-241, Cd-109, Co-57, Ce-139, Hg-203, Sn-113, Sr-85, Cs-137, Y-88, and Co-60 is available from many commercial source manufacturers who provide NIST traceable sources The gamma-ray energies of this mixed standard as well as some other commercially available NIST traceable radionuclides that are suitable for efficiency calibration (and energy and shape calibration) are shown in Table 1 As another example, an antimony-125/ europium-154,155 mixture from NIST (SRM 4275B or its replacement) has 19 major photopeaks between 100 and 1600 keV

TABLE 1 Radionuclides Useful for Energy Calibration

9.3.2 For environmental or low-activity samples (0.01 to 1

Bq/g), typically, 300 to 500 g of prepared soil are used If a

fixed volume is used, the mass will vary according to the

density High-density samples may cause significant

self-absorption of low-energy gamma rays and degrade the

higher-energy gammas Therefore, it is important to calibrate the

detector with standards of the same geometry, composition,

and density, or use appropriate attenuation and geometry

correction algorithms

9.3.3 Accumulate a gamma-ray spectrum using sealed,

cali-brated radioactivity standards until there are approximately

20 000 net counts (see Test MethodsE181or ANSI N42.14) in

each full-energy calibration peak provided that this does not

require an excessive amount of time To achieve reasonable

count times, smaller peak sizes may also be used Compare the

length of the count to the half-life of the radionuclide of

interest If the duration of the count is a significant portion of

the half-life (>10 %), a correction factor must be applied for

decay during the count Refer to Standard Test MethodsE181

or Ref (6 ) for additional information To correct the results to

the start of the measurement due to decay during spectrum

acquisition, use the following equation:

where:

K = multiplicative corrective for peak area or intensity,

λ = nuclide decay constant (in the same time units as T),

and

T = data collection elapsed clock time (that is, real time, not

live time)

9.3.4 The equation shown in 9.3.3 is not sufficient if the

count rate changes significantly during data acquisition, as

might happen if a short-lived nuclide is the main source of the

activity Other methods, such as the Virtual Pulser (25 ) and add

“N” Method (26 ) may be used for a varying count rate

situation

9.3.5 Correct the radioactivity standard source gamma-ray

emission rate for the decay from the time of standardization to

the start of data acquisition Many commercial and

noncom-mercial data analysis software packages will do this

automati-cally as part of the efficiency calibration

9.3.6 Calculate the full-energy peak efficiency, E f, as

fol-lows:

E f 5N p

where:

E f = full-energy peak efficiency (counts recorded per

gamma ray emitted),

N p = net gamma-ray count rate in the full-energy peak of

interest (counts per second), and

N g = gamma-ray emission rate (gamma rays per second)

9.3.7 If the standard source is calibrated for activity rather

than emission rate, the gamma-ray emission rate is given as

follows:

where:

A = number of nuclear decays per second, and

P g = emission probability for the gamma ray per nuclear decay

9.3.8 Many modern spectrometry systems are computerized, and the determination of the gamma-ray effi-ciencies is accomplished automatically at the end of an appropriate counting interval Refer to the manufacturer’s instructions for specific input requirements It is necessary for the user to determine the basis of the system analysis and its limitations

9.3.9 Plot the values for the full-energy peak efficiency (as determined in 9.3.6) versus gamma-ray energy The plot will allow the determination of efficiencies at energies for which standards are not available Many computerized systems pro-vide such plotting capabilities as part of the overall function-ality of the system Computerized systems also typically provide a variety of different calculational models to automati-cally calculate the efficiencies at any energy The calibration curve, regardless of whether it is calculated by hand or by a computerized system, generally should not be used for peak energies beyond the first and last calibration points If it is necessary to use the calibration curve in such a manner, one must pay particular attention to establishing appropriate uncer-tainties for any peak energies outside the calibration range Alternatively, calibrate using standards of the radionuclides of interest to obtain direct calibration factors for them without establishing an efficiency curve

10 Measurement Control

10.1 A properly run laboratory must have a measurement control program to verify that the detection system is in calibration See GuideC1009or Ref (27 ) for further guidance

on laboratory measurement control programs

10.2 As a minimum, the following periodic checks should

be made

10.2.1 Each day or prior to each measurement, energy, resolution, and efficiency response should be checked using at least two different gamma-ray energies If the energy calibra-tion slope and intercept are essentially unchanged, the energy calibration data are assumed to remain valid If an appreciable change in the slope or intercept is evident, the reason should be determined and corrected

10.2.2 Once the efficiencies for the various sample sizes, matrices, and source-to-detector distances have been established, it is not necessary to repeat the process unless there is a change in resolution or system configuration, or a new sample size, matrix, or geometry is added, or the detector has failed and has been returned to service after it has been repaired However, a complete check of the efficiency and energy calibrations should be done periodically (typically annually) Similar tests should be used to determine loss of

resolution or efficiency (28 ).

10.2.3 Ideally, a measurement of the room background should be made before and after any series of determinations 10.2.4 A measurement of a standard or sample with a known concentration to provide a measurement bias check Radioac-tive decay of the standard must be taken into account

10.2.5 Periodic replicate measurements of a standard or

sample to determine that the precision reported by the analysis

method, such as a computer program, is appropriate

10.3 It is recommended that control charts and other

peri-odic statistical analysis of the precision and bias data be used

11 Sample Measurements

11.1 After the spectrometer system has been set up and the

energy and efficiency calibrations performed, unknown

samples can be measured

11.2 Soil samples are collected by methods in accordance

with Practice C998 and prepared for analysis in accordance

with PracticeC999 An appropriate aliquot of soil is transferred

to the sample container (8.1) and positioned in the same

manner as was done during system calibration (Section9)

11.3 Measure the activity of the sample for a period of time

long enough to acquire a gamma-ray spectrum which will meet

the minimum acceptable counting uncertainty for the

radionu-clides of interest

12 Calculation

12.1 Ambient background peak areas must be subtracted if

their contributions are not negligible In many experiments, the

background may not affect the results but is still monitored to

ensure the integrity of the system The method presented here

is not the only acceptable one but is compatible with available

computational hardware and should be used to verify the

validity of commercial software

12.2 The underlying aim of this method is to subtract the

continuum or baseline from the spectral data where it underlies

a photopeak of interest For operator-directed calculations, the

choice of the baseline level may be straightforward The

simplest way, using a plot of the spectral data, is to draw a

straight line, using judgement and experience, that best

de-scribes the baseline The baseline data can be read directly

from the plot and subtracted A variety of computer programs

have accomplished this but details are not included in this

guide

12.3 Photopeaks lying on a sloping baseline, or one with

curvature, will be analyzed, regardless of method, with

in-creased uncertainty Use of data from these peaks should be

limited to those cases where there is no other alternative

Photopeaks that overlap with each other will also increase the

uncertainty of the final result In the case where use of

overlapping peaks cannot be avoided and software programs

are not available, the experimenter may estimate the areas by

assuming that the ratio of the peak areas is equal to the ratio of

the peak heights, but this may introduce a sizable error

Computer programs separating overlapping peaks with varying

degrees of success may be found in Refs (29 ), ( 30 ) The current

quality of peak reduction and deconvolution software available

from the major counting system manufacturers is adequate for

most situations

12.4 The radioactive decay process is governed by Poisson

statistics In Poisson statistics, the variance in N accumulated

events acquired with a detector is simply N The standard

deviation is the square root of the variance

12.5 The areas of well-resolved spectral peaks can be determined by summing the data above the underlying con-tinuum For narrow peaks, the continuum can be well-approximated by interpolation of the background line The peak area can then be calculated from the following equation:

C 5 P 2F N p

2 S B1

N B11

B2

where:

C = net peak area (counts),

P = total counts in the peak including the

continuum,

N p = number of channels summed for P,

B1 and B2 = number of counts in continuum regions on

either side of peak, and

N B1 and N B2 = number of channels summed to determine B1

and B2, respectively.

12.6 The variance in quantity f that is a function of n independent variables x is approximated by the following

equation:

σ 2~f!5(i51

n

S αf

αx iD2

12.7 Hence, the propagated variance of the area C is given

by the following expression (remembering that the variance of

P = P, variance of B1 = B1, and variance of B2 = B2):

σ 2~C!5 P1F S N p

2·N B1D2

·B1G1F S N p

2·N B2D2

·B2G (6)

where:

σ(C) = standard deviation of Peak Area C.

12.8 In order to determine radionuclide activity concentrations, the photopeak areas, corrected for background and interferences, are divided by the count time and efficiency for the energy of the gamma ray being calculated to yield gammas per second for the peak of interest If, as is the case for some radionuclides, the gamma-ray abundance is 100 %, division by the detector efficiency converts counts per second for the photopeak of interest to decays per second (bequerels) for the nuclide If not, the gammas per second are converted to disintegrations per second by dividing the gammas per second

by the gammas per disintegration, for the nuclide and photo-peak of interest The results are then corrected for sampling or decay, or both, as demanded by the application The activity of

a particular radionuclide may be calculated using the following equation:

where:

A = activity concentration, Bq/g,

ε = efficiency of the spectrometer for the gamma ray of

interest,

K µ = correction factor to accommodate for the attenuation

in the sample matrix compared to the attenuation in the matrix of the standard source,

B = number of gamma rays emitted per decay,

e –λTw = decay correction,

W = weight of sample, g,

T = count time, s, (live time), and

T w = decay time

The energies, half-lives, and gammas per disintegration for

typical radionuclides that might be present in soil samples are

available in Refs (14-18 ).

12.8.1 The activity of the ith radionuclide can also be

calculated as follows:

A i 5 k i C

where:

k i = calibration factor for the ith radionuclide.

13 Precision and Bias

13.1 Precision:

13.1.1 Precision of the method is influenced by random

counting uncertainties, and interferences in the spectra of the

individual components with each other as well as the sampling

uncertainty The more complex the spectrum, the greater are

the errors, and in general, major components can be determined

more precisely than minor ones Precision may be improved

with increased counting time and by taking as large a sample of

the soil volume being analyzed as possible Variations in

sample vial geometry and positioning will affect the precision

of the measurement as well

13.1.2 To illustrate typical precision of a gamma-ray

spec-trometry system, a check source was counted ten consecutive

times without removing the source from the detector system

The ten single operator counts of ten minutes each provide a

measure of system repeatability The results are listed inTable

2

13.1.3 The same source was used and another set of 10

measurements of 10 min each were made on successive days to

capture more sources of variation This process involved the

normal day-to-day system operational checks and should

provide a measure of the variability of operating procedures

The results are listed inTable 3

13.2 Bias:

13.2.1 The calibration of standard sources, including errors

introduced in using a standard radioactive solution or aliquot

thereof, to prepare a working standard for bias correction may

result in a bias The full-energy peak efficiency at a given energy determined from the calibration function may introduce

a bias

13.2.2 The single-operator bias of a gamma-ray spectrom-etry system was estimated by measuring the NIST Rocky Flats Soil Number 1 (SRM 4353) and the NIST River Sediment (SRM 4350 B) six different times The results are shown in

Table 4

13.3 Sources of Error:

13.3.1 Variation of the physical geometry of the sample and its relationship with the detector will produce both qualitative and quantitative variations in the gamma-ray spectrum To adequately account for these geometry effects, efficiency cali-brations (and occasionally also energy calicali-brations) should be designed to duplicate all conditions including source-to-detector distance, sample shape and size, and sample matrix When it is not possible to have a calibration source that duplicates the sample matrix, the difference between the calibration matrix and the sample matrix as well as its height can be corrected for by a transmission measurement and a calculation and use of a geometry dependent attenuation factor

(see Ref (31 )).

13.3.2 Since some spectrometry systems are calibrated for various size sources at many different source-to-detector distances, a wide range of activity levels can be measured by the same detector For high-activity samples (for example,

>106Bq), which may have resulted from a spill or accident, a large source-to-detector distance (for example >1 m) may be used The large source-to-detector distance for high-activity samples will reduce the overall rate and thus minimize the random summing problems in the spectrum A larger source-to-detector distance will also remove coincidence summing, if present, regardless of count rate

13.3.3 Electronic problems, such as loss of resolution and random summing, may be minimized by keeping the overall count rate below 2000 counts/s For most soil samples, a high count rate is not a problem Some care may be needed in preparing or purchasing calibration standards so that their count rate in the measurement geometries stays below the desired limit Total counting time is governed by the radioac-tivity of the sample, the detector-to-source distance, and the acceptable Poisson counting distribution uncertainty

TABLE 2 Precision of Repeated Gamma-Ray Measurements in Counts per Minute (CPM) Without Removing the Sample Between

Measurements

Measurement

13.3.4 The density of the sample is another factor that can

affect quantitative results Errors from this source can be

avoided by preparing the standards for calibration in matrices

with a composition and density comparable to the sample being

analyzed The important factor is the linear attenuation which

is the product of the density and the mass attenuation

Attenuation correction methods to correct for differences in the

densities of the calibration source and actual samples, such as

the use of a transmission source, are acceptable to use if they

can be demonstrated to give good results

14 Keywords

14.1 coincidence summing; gamma-ray; high-purity

germa-nium; HPGe; photopeak; Poisson; radionuclides; shield; soil

TABLE 3 Precision of Reproducibility of Gamma-Ray Measurements in Counts per Minute (CPM) While Removing the Sample from the

Measurement Position Between Measurements

Measurement

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TABLE 4 Results of NIST Rocky Flats Soil Number 1 (SRM 4353) and NIST River Sediment (SRM 4350 B) (Each measured value is

an average of 6 measurements of 1000 min each The estimated measured uncertainty includes only the counting statistics The

NIST uncertainty is the total uncertainty.)

SRM 4353 (Bq/ g) Radionuclide Measured

Value

NIST Value

Ratio Measured/NIST

Cs-137 0.0190 ± 0.0010 0.0176 ± 0.0008 1.08 Ra-226 (Bi-214) 0.0430 ± 0.0030 0.0430 ± 0.0028 1.00 Th-228 (Tl-208) 0.083 ± 0.002 0.0708 ± 0.0036 1.17 Th-232 (Ac-228) 0.0690 ± 0.005 0.0693 ± 0.0035 1.00

SRM 4350 B (Bq/g) Co-60 0.00464 ± 0.0024 0.00464 ± 0.00023 1.00

Cs -137 0.031 ± 0.002 0.0290 ± 0.0018 1.07 Eu-152 0.037 ± 0.003 0.0305 ± 0.0012 1.21 Eu-154 0.0035 ± 0.0022 0.00378 ± 0.00057 0.93 Ra-226 (Bi-214) 0.034 ± 0.002 0.0358 ± 0.0036 0.95

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Nonde-structive Assay of Nuclear Materials, (D Reilly, N Ensslin, H.

Smith, Jr., and S Kreiner, eds.) NUREG/CR-5550, 1991.

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